1. Field of the Invention
The present invention relates to an optical receiver applicable in the optical communication.
2. Related Prior Art
A SFP (Small Form-Factor Pluggable) link has been widely spreading as one type of optical transceivers. For instance, a home page on the internet, “INF-8074 specification for SFP (Small Form Factor Pluggable Transceiver Rev. 1.0”, May 12, 2001; ftp://ftp.seagate/com/sff/ING-8074.pdf; by the SFF committee, has disclosed a specification of such a transceiver.
The SFP transceiver is requested to have a function that, when the optical input signal becomes less than intensity specified by the standard due to the breakdown or the miss connecting of the optical fiber, or due to the failure of the optical transmitter, the transceiver sets a LOS (Loss Of Signal) alarm. In addition to the function mentioned above, the optical transceiver is necessary to reset the LOS alarm when the optical input signal recovers and the intensity thereof exceeds a specific value. The specification of the SFP transceiver requests that the setting and the resetting of the LOS alarm is carried out within 100 μs from the disappearance or the recovery of the optical signal.
Conventional optical receiver detects the LOS state by the amplitude of AC components of the electrical signal converted from the optical input signal. Accordingly, when an optical signal is input in step-like at the LOS state, the receiver may miss the setting of the LOS alarm because an electronic circuit within the receiver miss-operates at a leading edge of the step-like optical signal, which temporarily resets the LOS alarm. Moreover, when the optical signal configures, what is called, a burst mode signal with substantial intensity, a situation, in which the optical signal is regarded to be interrupted, may occur at the leading edge of the burst signal, which leads the optical receiver to set the LOS alarm.
Specifically, the optical receiver, in particular, a pre-amplifier in the optical receiver, generally provides an auto-gain-control (AGC) function to maintain an amplitude of the output signal thereof constant. In such an optical receiver, when the optical signal recovers from the LOS state, that is, at leading edge of the burst signal, the gain of the pre-amplifier set by the AGC gets into an excess state until an optimum value is reached from the maximum. Consequently, the pre-amplifier generates an output with a step-like form just after the recovery of the optical input signal because the pre-amplifier saturates the output thereof in the highest level. Because the LOS alarm is derived from the AC component of the output from the pre-amplifier, the receiver temporarily repeats the reset and set of the LOS alarm following the output behavior of the pre-amplifier, and finally, resets the LOS alarm by the gain of the pre-amplifier stable in an optimum value. Thus, the conventional optical receiver does not provide a function to reliably detect the recovery of the optical signal from the LOS state.